The present invention relates to devices for providing a fluid-tight seal or block in a tubular member such as a conduit or nozzle or the like. The invention has particular application to providing a dam or seal in the primary fluid nozzles of a nuclear steam generator.
A typical nuclear steam generator comprises a vertically oriented shell or vessel. A tube sheet toward the lower end of the vessel supports a tube bundle comprising a plurality of tubes, each generally in the shape of an inverted U. The portion of the vessel beneath the tube sheet is divided by a partition into inlet and outlet plenums which are respectively disposed immediately beneath the vertical legs of the tube bundle. The vessel is provided with primary fluid inlet and outlet nozzles, respectively communicating with the plenums, each nozzle projecting outwardly of the vessel and commonly having a frustoconical portion and a circularly cylindrical portion. The nozzles are connected by conduits to an associated nuclear reactor vessel.
Primary fluid, having been heated by circulation through the reactor, enters the steam generator vessel through the primary inlet nozzle to the inlet plenum and from there flows upwardly through the tube bundle to the outlet plenum and then back through the outlet nozzle to the reactor core. The tube bundle above the tube sheet is immersed in a secondary fluid, such as water, the tube bundle serving as a heat exchanger for converting the secondary water to steam, which is then used for generating electricity in the usual manner.
Periodically, it is necessary to shut down the reactor for refueling. This is usually a convenient time for servicing the nuclear steam generator. In this regard, the reactor is drained to below the level of the inlet and outlet nozzles of the steam generator vessel. The tube bundle and inlet and outlet plena are also drained and dams are then installed in the inlet and outlet nozzles to block them. The steam generating vessel can then be treated in a chemical decontamination procedure without contaminating the primary fluid conduits and the nuclear reactor.
Heretofore, a number of different types of fixed or static nozzle dams have been utilized. Most of these dams involve permanent modification of the nozzle, either by drilling of holes therein or the mounting of attachment structure thereon, these techniques entailing man-rem exposure and affecting the flow of primary fluid through the nozzle. It is also known to utilize expandable bags or diaphragms or the like which are inserted in the nozzle and expanded to plug the nozzle. But such prior expandable seals do not adhere particularly well to the nozzle inner surface.
One type of nozzle dam, utilized by Westinghouse Electric Corporation, involves the use of one or more seal assemblies, each including a foldable circular seal plate encircled with an inflatable seal which is disposable in frictional engagement with the nozzle wall. Certain embodiments of this type of seal are disclosed in the copending U.S. application of John J. Wilhelm, Paul H. Dawson, Arthur W. Kramer and Gregory L. Calhoun, U.S. Pat. Ser. No. 605,354, filed Apr. 30. 1984 entitled NON-BOLTED RINGLESS NOZZLE DAM.
Another version of the Westinghouse nozzle dam incorporates two or more of the circular foldable seal assemblies, interconnected by a central tubular coupling, incorporating a quick-disconnect which is operable from outside the seal assemblies. The coupling is threadedly engaged with each of the two interconnected seal assemblies. While this arrangement operates well in most applications, any torque on either of the two seal assemblies tends to cause relative rotation of the seal assemblies, resulting in unscrewing of the coupling and attendant loosening of the seal. Such loosening results in excessive leakage.
Furthermore, the seal assemblies of this arrangement could move as a result of pressure changes on opposite sides thereof, improper seating and the like and, as a result of such movement, the two seal assemblies may not stay parallel, which also would result in poor sealing and resultant leakage. Furthermore, this non-parallel condition could cause the center tubular coupling to be bent, which would interfere with the operation of the quick disconnect mechanism.